1. Field of the Invention
The invention relates to epicyclic gearboxes with particular application to aircraft gas turbine engines with counter-rotatable fans.
2. Description of Related Art
An aircraft gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor or spool. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor. The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor or spool. The low pressure shaft extends through the high pressure rotor.
Some fan jet engines have been designed with counter-rotating fans and some with counter-rotating fans and/or boosters or low pressure compressors. U.S. Pat. Nos. 4,790,133, 4,860,537, 5,307,622 and 6,732,502 disclose counter-rotating low pressure turbines (LPT) that power counter-rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan. There are also various designs for counter-rotating fan engines that use gearboxes to effect counter-rotation of the fans and boosters. Counter-rotating fans, boosters, and turbines greatly enhance the engine's fuel efficiency. U.S. patent application Ser. No. 11/555,042 discloses using a low-pressure turbine to drive counter-rotating forward and aft fans through an epicyclic gearbox.
Size, weight, and reliability of an epicyclic gearbox depends a great deal on planet bearing loads, life requirements, and gear tooth stresses. It is highly desirable to significantly reduce the planet bearing loads of a counter-rotating epicyclic gearbox in order to increase gearbox reliability. It is also highly desirable to reduced loads enabling the use of smaller bearings which reduces weight and improves life of the gearbox. Smaller bearings also decrease the heat generation of the gearbox which results in an efficiency improvement and reduced oil flow requirements.
It is also highly desirable to reduce or eliminate gear misalignment that results from the twisting due to torque. Improved alignment reduces or eliminates the need to modify gear tooth shape to compensate for deflection, resulting in improved gear durability and reduced gear tooth stresses.